In recent years, with the development of portable devices such as personal computers and mobile phones, there is an increasing need for batteries as their power supplies. Batteries to be used for such purposes are required to have a high energy density and excellent cycle characteristics. Against such requirements, for both the positive electrode and the negative electrode, high-capacity active materials are being newly developed. Among others, an elemental, oxide, or alloy form of silicon (Si) or tin (Sn), which can provide a very high capacity, is regarded as a promising negative-electrode active material.
However, when a negative electrode for a lithium secondary battery is constructed by using such negative-electrode active materials, there is a problem in that the negative electrode will be deformed through repetition of charging and discharging. The aforementioned negative-electrode active materials undergo significant volumetric changes when reacting with lithium ions. Therefore, at the time of charging and discharging, the negative-electrode active material will undergo significant expansion/contraction due to reactions of insertion and desorption of lithium ions with respect to the negative-electrode active material. Therefore, in a negative electrode of a structure in which an active material layer containing a negative-electrode active material as mentioned above is formed on a current collector, when charging and discharging are repeated, a large stress may occur near the interface between the active material layer and the current collector to cause strain, thus resulting in wrinkles and breaks of the negative electrode, peeling of the active material layer, and so on. Moreover, due to the expansion stress of the negative-electrode active material, the active material layer may be cracked, or a portion of the active material layer may become pulverized. Furthermore, when the negative electrode is strained and deformed, a space may be created between the negative electrode and the separator, so that the charging and discharging reaction may become nonuniform, thus locally deteriorating the battery characteristics. Therefore, it has been difficult to obtain a lithium secondary battery having sufficient charge-discharge cycle characteristics by using the aforementioned negative-electrode active material.
In order to solve such problems, a construction has been proposed in which spaces for alleviating an expansion stress of the negative-electrode active material are provided in the negative electrode, thereby suppressing deformation of the negative electrode.
Patent Document 1 discloses an electrode structure in which a plurality of pillar-like members are regularly arrayed on a current collector, the pillar-like members being made of an active material which forms an alloy with lithium. With this electrode structure, at the time of charging, each pillar-like member expands so as to fill the voids between pillar-like members, so that the stress acting on the entire negative electrode can be alleviated, whereby strain of the negative electrode and peeling of the active material can be suppressed.
Patent Document 2 proposes a negative electrode structure in which a plurality of pillar-like active material particles, made of a negative-electrode active material, are formed on a current collector. In Patent Document 2, vapor deposition of the active material is performed from a direction which is tilted with respect to the normal direction of the current collector surface (oblique vapor deposition), thereby forming active material particles whose longitudinal direction is tilted with respect to the normal direction of the current collector surface. With this structure, too, spaces for allowing the active material particles to expand can be obtained between active material particles, whereby deformation of the negative electrode due to an expansion stress can be suppressed.
On the other hand, a construction has been proposed which reduces deformation of a negative electrode by dispersing the expansion stress of the negative-electrode active material. For example, Patent Document 3 discloses a negative electrode of a structure such that, in an active material layer which mainly contains silicon and oxygen, the oxygen content is varied so that layers having a low expansion rate upon charging and layers having a high expansion rate upon charging are alternately stacked. In the negative electrode of Patent Document 3, since layers having a low expansion rate are inserted between layers having a high expansion rate, the expansion rate of the entire active material layer is kept small, and the layered structure allows the stress due to an expansion of the layers having a high expansion rate, i.e., a smaller oxygen content, to be dispersed. As a result, it is possible to suppress peeling and pulverization of the active material layer due to an expansion stress of the active material layer, and reduce deterioration of the charge-discharge cycle characteristics.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-127561    [Patent Document 2] Pamphlet of International Publication No. 2007/015419    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2006-196447